My father has a 90 rx-7 and we are having overheating problems...we have replace the waterpump, thermostat, and had the radiator pressure tested...here is the problem, we can drive the car and even let it sit running for hours, and nothing, and then sometimes all of the sudden boom its overheating and the coolant is overflowing the coolant resivoir...the mechanic called the local rotary shop and right away they said that it needs a new motor and that the cool seals are leaking, now the car does not smoke at all, no white smoke no nothing, it even has straight pipe, so i am sure we would have noticed it....so if anyone has any ideas please help me, we are getting desperate, and i am about to shoot the damn car lol...

Peace
Buddy

 

Check if its overheating when revved really high for some time (like when racing). I had the same problem, and after numerous attempts to fix it (changed everything twice ), I thought I had a small hole in the water seals, which let very small amounts of exhaust gases in the rotors, without letting the coolant seep through....

....as it turned out, the overheating was caused by a) cracked exhaust manifold and turbo, which let exhaust gases into the engine bay, and b) a missing lower tension bolt

In your case, I would make sure that thermostat and cap are OK, and then change all the small rubber hoses and the two gaskets (one in the filler neck and one in the pump). Small cost, easy to do. If problem remains, then test coolant for exhaust gases, and/or drill down to exhaust gaskets and manifold...